1. Technical Field
The invention relates generally to an improved vehicle suspension system. More particularly, the invention relates to air spring suspension systems for land vehicles which include a parallelogram kinematic movement. Specifically, the invention relates to a parallelogram suspension system which is roll stable and resistant to lateral deflection.
2. Background Information
Suspensions are available in the prior art which utilize air springs to provide a comfortable ride, even for large over-the-road trucks and other heavy vehicles. The air springs are typically used in industrial vehicle single axle units, or tandem arrangements of two or more axles which are either driven or non-driven.
One drawback of air spring suspensions is that an air spring is essentially an air inflated bag and requires auxiliary stabilization. An air suspended axle must have separate independent mechanical location and attitude controls and stabilized components or it will not function effectively. Absent stabilization, the air spring will extend to its maximum length or width in the direction of least resistance. Also, lateral loading from cornering or negotiating uneven terrain will cause a vehicle supported on unstable air springs to lean and possibly roll-over.
A significant number of air spring suspensions have been developed which to a greater or lessor extent, control axle location and attitude. A number of suspensions that have been developed are roll rigid, while others are roll flexible, each generally being designed for a specific application. The most common roll rigid configuration is the trailing beam type suspension, most of which use the axle as a torsion rod to provide roll rigidity.
Another type of suspension which has been developed is the parallelogram suspension which is not inherently roll rigid, and does not inherently provide lateral stiffness. Again, ancillary devices such an anti-roll bars, track bars or guide mechanisms have been utilized to stabilize typical parallelogram designs. As such, parallelogram type suspensions, even with the ancillary devices attached, were often only suitable for low center of gravity loads, or on specialized vehicles stabilized by other vehicle suspension mechanisms.
Trailing arm suspensions are brake reactive. That is, when the vehicle brakes are applied, the suspension will tend to compress thereby altering axle loading and potentially reducing the suspension and brake effectiveness. Similarly, when the brakes are applied as the vehicle moves in reverse, the suspension will tend to raise up, and pivot about the single trailing arm pivot, again altering axle loading and reducing the suspension and brake effectiveness. Further, most trailing arm suspensions suffer from dock walk such that they move toward or away from the loading dock as the suspension moves up or down with the brakes locked. This movement is caused from air draining off the air springs, or as a result of loads added to or removed from the vehicle, or the temperature changes that occur as the trailer remains parked by the dock. Dock walk occurs, in part, because between the fully compressed to the fully expanded position of the air spring, the free end of the trailing arm travels a significant linear distance as a result of movement about a single pivot point. As such, with the brakes locked, as they would be while parked at a dock, the tires also rotate with the trailing arm and cause forward and rearward vehicle motion. Similarly, trailing arm suspensions do not utilize the air springs full capacity as the air spring plates are not parallel in extreme operating positions, again as a result of the trailing arm pivoting about a single pivot point. The rear of the air spring is thus fully extended long before the forward part of the air spring.
Parallelogram suspensions were developed to solve a number of the problems associated with trailing arm type suspensions. However, parallelogram suspensions create problems not present in trailing arm type suspensions. Specifically, parallelogram suspensions are not inherently roll rigid nor do they inherently provide lateral stiffness. Parallelogram suspensions have been found to be a significant advancement over the prior art as they provide a relatively stable, safe, and comfortable ride for all types of loads. Some of these parallelogram suspensions are included in U.S. Pat. Nos. 4,114,923, 4,132,432 and 4,309,045.
Advantages of the parallelogram stabilized air spring suspensions include that the air suspended axle in a parallelogram suspension moves thru a very short linear distance and has no rotational motion between the loaded and unloaded positions which reduces the problem of dock walk inherent in trailing arm type suspensions.
Further, the parallelogram stabilized suspension permits the air spring's full capacity to be utilized as the top and bottom air spring plates remain substantially parallel throughout the axle lift operation. Specifically, when the air spring is mounted on a moving link of the parallelogram it allows the utilization of the air springs full lift capability when compared to the typical trailing arm design where the air spring travels in an arc and "fans" open stretching the rearmost internal reinforcing fibers of the spring while not utilizing even the full travel of the forward part of the air spring.
A further advantage of the parallelogram suspension is its inherent ability to maintain a constant caster angle for steerable or caster steering axles which are often utilized in auxiliary axle suspensions for tractors and trailers.
The parallelogram suspension inherently provides the above advantages, and also locates the axle relative to the longitudinal axis of the vehicle by controlling the forward and rearward motions of the axle relative to the frame. Moreover, a parallelogram suspension also controls the path which the air spring follows as it operates to take up irregularities in the road surface. However, the parallelogram suspension alone does not stabilize the air spring. Specifically, the parallelogram itself does not provide lateral stability to the suspension system.
Lateral forces act on a suspension system in a variety of ways with the most common being that lateral forces act on a suspension as the vehicle negotiates a turn. As the vehicle turns, shear stresses act between the tire and the road surface causing a lateral stress to be transferred through the tire-wheel assembly to the axle. The axle, being rigidly attached to the suspension, transfers the lateral forces into the parallelogram causing it to laterally deflect. This lateral deflection can be extreme and substantially limits the usage of parallelogram suspensions. Lateral force may be strong enough under certain loading conditions that the tires contact the vehicle frame rails.
It is thus necessary to provide mechanical means for controlling lateral forces on the suspension and its various members. One typical suspension where lateral forces are mechanically controlled is shown in U.S. Pat. No. 3,140,880 in which air springs are disposed between two vertically swinging control arms to which the axle is also attached. One feature of this suspension is that much of the lateral force is controlled by a strong, relatively rigid attachment between the axle and the control arms. As such, the lateral force is taken up by the attachment between the control arm and the axle. While this prior art suspension system presumably functioned for the purpose for which it was intended, it suffered from dock walk, brake reactivity, and it did not utilize the full lift potential of the air spring. Moreover, it is desirable to provide for greater flexibility between the axle and the control arms, while still maintaining sufficient lateral stability and thus increase the suspensions roll stability. Thus, the second problem inherent in parallelogram air spring suspensions is that they are not roll stable.
Roll instability refers to the lack of sufficient counteracting forces operating on the ends of an axle allowing one end of the axle to raise relative to the frame a distance greater than the other end of the axle. Roll instability is encountered when the vehicle frame tilts or rolls relative to the axle; for example, when the vehicle negotiates a turn such that the centrifugal and acceleration forces reduce the downward forces acting on the inside wheel of the turn, and increase the downward force acting on the outside wheel of the turn. Roll instability can also be realized when the axle moves relative to the frame; for example, during diagonal axle walk.
Diagonal axle walk occurs when the axle moves relative to the vehicle frame which occurs when the wheels at the opposite ends of the axle encounter unlike irregularities in a road or off-the-road surface, such as when one wheel rides over a curb. As the wheel rides over the curb, an upward force acts on that wheel, and a counteracting downward force acts on the wheel not riding over the curb. If the suspension is unable to provide flexibility between the axle and the frame as the tire-wheel assembly travels over the curb or ground irregularity, or alternatively to provide the same resilience or flexibility between the axle and the frame as the vehicle negotiates a turn, the suspension will be too roll rigid, and may cause axle breakage and over-stress vehicle components. Roll rigid suspensions are used to stabilize high center of gravity vehicles like highway trailers, and are most critical in applications such as tank or dump trailers and vans having high volume boxes. In these applications, only enough roll compliance is permitted to allow the axle suspension combination to negotiate uneven terrain without unduly stressing the vehicle frame or axle. Typically, the roll angles of axle to frame are 2 to 3 degrees in roll rigid environments. That is, if all the load were transferred to the tire or tires on one side of the vehicle and the tire or tires on the other side of the vehicle are completely off the ground, the angle of the axle relative to the frame reaches only about 2 to 3 degrees for a typical roll rigid suspension.
Conversely, roll flexible suspensions are used on low height vehicles and multi-axle vehicles which are stabilized by only some of the suspensions and the added axles merely increase the load carrying capacity of the vehicle. In applications where tractive effort is paramount, the suspension must be flexible to allow the tires to remain in contact with the ground. Specifically, if a given suspension is roll flexible, so that the vehicle may have a larger total vehicle weight, the tire must remain in ground contact to assure that weight is transmitted to the ground through the tire.
Regardless of whether a roll rigid or roll flexible suspension is required, the suspension itself must be roll stable for the safety reasons set forth hereinabove.
Attempts have been made to provide additional resistance to lateral forces while simultaneously allowing the frame to "roll" in a controlled manner relative to the axle without interfering with the vertical forces controlled by the air springs. Prior attempts to provide additional roll resistance include the addition of stabilizer bars, roll bars or torsion bars secured between the suspension and the frame, or by stiffening the connection between the axle and the control arm as described above. One such suspension is shown in U.S. Pat. No. 5,083,812.
Such improvements, however, may nevertheless affect the handling and ride of the vehicle, and transfer the load caused by the lateral forces to the frame thereby over-stressing vehicle components. Such systems are frequently more complex, having many moving components, and may also have limited application, especially where the vehicle center of gravity is over a predetermined height.
A roll stable parallelogram suspension which is resistent to lateral forces would have a variety of uses. The parallelogram suspension has not been used in a roadrailer application as the vertical distances the suspension travels magnifies the affect of lateral forces acting on the suspension. In the roadrailer application, the axle must be moved between three separate positions: a first ground engaging position when the roadrailer suspension is operating in highway mode, a second ground engaging position when the trailer is raised to engage a rail bogie in coupling mode, and a rail mode wherein the tires are lifted above the railing. The size of the air spring necessary to move the suspension between these three positions made the use of other parallelogram suspensions unrealistic as the affects of lateral forces and roll instability could not be overcome while trailing beam designs require more airspring travel decreasing ground clearance and increasing cost and weight.
Roadrailer suspensions utilize a lifting mechanism which may either be an air spring, or a mechanical spring of the leaf or coil variety. The conventional axle lifting mechanism comprises one or more stressed mechanical springs such as coil springs or leaf springs acting directly between the vehicle frame and axle. When air is relieved from the air springs, the mechanical springs raise the axle. The mechanical springs, in their condition of diminished stress when the axle is fully raised, must still exert sufficient force to support the weight of the axle and tire-wheel assemblies such that the wheels remain in the raised position. When the air springs are pressurized, the wheels are forced downwardly into ground engagement overcoming the mechanical spring force.
Therefore, a need exists for a road-railer suspension which is parallelogram stabilized and is roll stable, but which is also resistent to lateral forces.